The invention relates to a process for the preparation of xcex1-dicarbonyl compounds (xcex1-diketones), in particular for the preparation of 2,3-pentanedione.
2,3-Pentanedione is a constituent of the butter aroma and is therefore prepared industrially as a synthetic aroma substance. It is suitablexe2x80x94like other xcex1-dicarbonyl compoundsxe2x80x94also as a synthesis building block for the preparation of further fine chemicals and active ingredients.
2,3-Pentanedione can be prepared, for example, by condensation of lactic acid (2-hydroxypropanoic acid) (U.S. Pat. No. 5,831,130). The condensation reaction is carried out at temperatures of about 300xc2x0 C.
U.S. Pat. No. 3,153,066 describes the ozonolysis of ketones in the presence of selenium dioxide for the preparation of xcex1-dicarbonyl compounds.
It was an object of the present invention to provide a simple process for the preparation of xcex1-dicarbonyl compounds.
It has been found that xcex1-dicarbonyl compounds can be obtained by reacting xcex1,xcex2-unsaturated ketones with ozone.
The invention thus relates to a process for the preparation of xcex1-dicarbonyl compounds of the formula 
in which R1 and R2, independently of one another, are C1-6-alkyl. The process is characterized in that an xcex1,xcex2-unsaturated ketone of the formula 
in which R1 and R2 have the meaning given above, and R3 and R4, independently of one another, are hydrogen, C1-6-alkyl or di-C1-6-alkylamino, is reacted with ozone, and the reaction mixture is then worked-up reductively.
xe2x80x9cC1-6-Alkylxe2x80x9d is to be understood here and in the text below as meaning all linear or branched alkyl groups having 1-6 carbon atoms, such as, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, pentyl, isopentyl, tert-pentyl, neopentyl, hexyl or isohexyl.
The radical R1 is preferably methyl. The radical R2 is preferably ethyl.
The reaction with ozone advantageously takes place in a suitable solvent at temperatures in the range from xe2x88x92100xc2x0 C. to 30xc2x0 C. Suitable solvents are, for example: alcohols, such as, for example, methanol, ethanol, propanol, isopropanol, butanol, pentanol or hexanol; organic acids, such as, for example, acetic acid or customary inert solvents, such as, for example, hexane, dichloromethane, ethyl acetate. It is, however, also possible to use mixtures of the above solvents. Particular preference is given to methanol or a mixture of methanol and dichloromethane.
An ozone-containing stream of gas, such as, for example, an air/ozone mixture or an oxygen/ozone mixture, is introduced into the reaction mixture. The stream of gas preferably comprises 0.5 to 10% by weight of ozone.
The ozonolysis is complete as soon as the reaction mixture turns blue as a result of excess ozone.
When the reaction is complete, excess ozone is advantageously removed by introducing inert gas, for example by introducing nitrogen.
The ozonide formed is worked-up reductively in accordance with known methods, for example by adding dimethyl sulphide, sulphur dioxide, trimethyl phosphite, thiourea, zinc dust in acetic acid, sodium hydrogen sulphite or by means of catalytic hydrogenation.
The starting compounds of the formula (II) can be prepared by known methods.
For example, in a Mannich reaction, an appropriate ketone can be reacted with formaldehyde and dialkylamine. The resulting Mannich base is then heated, and the dialkylamine cleaves off and the xcex1,xcex2-unsaturated ketone of the formula II is formed.
xcex1,xcex2-Unsaturated ketones of the formula II can also be prepared by aldol addition from the appropriate ketones by reaction with acetaldehyde or paraldehyde.
Likewise, the reaction of ketones with xe2x80x9cGold""s reagentxe2x80x9d [3-(dimethylamino)-2-azaprop-2-en-1-ylidene]dimethyl-ammonium chloride leads to xcex1,xcex2-unsaturated ketones.
The examples below illustrate the process without representing a limitation.
All yields were determined by GC analysis of the reaction mixture, and where the yields are given in percentages, these are mole percentages.